A Design Guide Part and Mold Design Engineering Polymers PDF

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Engineering Polymers Part and Mold Design THERMOPLASTICS A Design Guide INTRODUCTION A product of the Bayer Design The manual focuses primarily on This publication Contact your Bayerwas sales written to assist representative Engineering Services Group, this manual plastic part and mold design, but a...

Description

Engineering Polymers

Part and Mold Design THERMOPLASTICS

A Design Guide

INTRODUCTION

A product of the Bayer Design Engineering Services Group, this manual is primarily intended as a reference source for part designers and molding engineers working with Bayer thermoplastic resins. The table of contents and index were carefully constructed to guide you quickly to the information you need either by topic or by keyword. The content was also organized to allow the manual to function as an educational text for anyone just entering the field of plastic-part manufacturing. Concepts and terminology are introduced progressively for logical cover-to-cover reading.

The manual focuses primarily on plastic part and mold design, but also includes chapters on the design process; designing for assembly; machining and finishing; and painting, plating, and decorating. For the most part, it excludes information covered in the following Bayer companion publications: Material Selection: Thermoplastics and Polyurethanes: A comprehensive look at material testing and the issues to consider when selecting a plastic material. Joining Techniques: Includes information and guidelines on the methods for joining plastics including mechanical fasteners, welding techniques, inserts, snap fits, and solvent and adhesive bonding. Snap-Fit Joints for Plastics: Contains the engineering formulas and worked examples showing how to design snapfit joints for Bayer thermoplastic resins.

Contact your Bayerwas sales representative This publication written to assist Bayer's customers in the design and for copies of these publications. manufacture of products made from the Bayer line of thermoplastic This publication was written engineering resins. Thesespecifically resins include: to assist our customers in the design and manufacture of products made from the - Makrolon® polycarbonate Bayer line of thermoplastic engineering - Apec® high-heat polycarbonate resins. These resins include: - Bayblend® polycarbonate/ABS blend ® Polycarbonate •- Makrolon Makroblend® polycarbonate/ polyester blend ® High-Heat Texin® and Desmopan® •- Apec Polycarbonate thermoplastic polyurethane ® Polycarbonate/ •For Bayblend information on these materials, ABS Blend please call 1-800-662-2927 or visit http://www. •BayerMaterialScienceNAFTA.com. Makroblend® Polycarbonate Blend

• Triax® Polyamide/ABS Blend The following additional products highlighted in this publication are now • Lustran® and Novodur® ABS part of LANXESS Corporation: ® SAN •- Lustran Cadon® SMA - Centrex® ASA, AES and ASA/AES polymers •weatherable Cadon® SMA - Durethan® polyamide 6 and 66, and amorphous polyamide • Centrex® ASA, AES and ASA/AES - Lustran® and Novodur® ABS Polymers - Weatherable Lustran® SAN - Pocan® PBT polyester ® Polyamide 6 and 66, •- Durethan Triax® polyamide/ABS blend and Amorphous Polyamide For information on these products, please ®call LANXESS in North •America Texin and Desmopan® at 1-800-LANXESS or visit: Thermoplastic Polyurethane http://techcenter.lanxess.com/sty/ for •americas/en/home/index.jsp Pocan® PBT Polyester styrenic resins http://techcenter.lanxess.com/scp/ americas/en/home/index.jsp for polyamide resins

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Most of the design principles covered in this manual apply to all of these resins. When discussing guidelines or issues for a specific resin family, we reference these materials either by their Bayer trade names or by their generic polymer type. The material data scattered throughout the chapters is included by way of example only and may not reflect the most current testing. In addition, much of the data is generic and may differ from the properties of specific resin grades. For up-to-date performance data for specific Bayer resins, contact your Bayer sales representative or refer to the following information sources: Bayer Engineering Polymers Properties Guide: Contains common single-point properties by resin family and grade. Bayer Plastics Product Information Bulletin: Lists information and properties for a specific material grade.

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Bayer CAMPUS: Software containing single and multi-point data that was generated according to uniform standards. Allows you to search grades of Bayer resins that meet a particular set of performance requirements. www.bayer.com/polymers-usa: Bayer Web site containing product information on-line. This manual provides general information and guidelines. Because each product application is different, always conduct a thorough engineering analysis of your design, and prototype test new designs under actual in-use conditions. Apply appropriate safety factors, especially in applications in which failure could cause harm or injury.

In addition to design manuals, Bayer Corporation provides design assistance in other forms such as seminars and technical publications. Bayer also offers a range of design engineering services to its qualified customers. Contact your Bayer sales representative for more information on these other services.

TABLE OF CONTENTS

Chapter 1 PART DESIGN PROCESS: CONCEPT TO FINISHED PART

Chapter 2 GENERAL DESIGN

7

Design Process

19 Wall Thickness

8

Defining Plastic Part Requirements

22 Flow Leaders and Restrictors

8

Mechanical Loading

24 Ribs

8

Temperature

24

8

Chemical Exposure

24

Rib Thickness

8

Electrical Performance

26

Rib Size

8

Weather Resistance

27

Rib Location and Numbers

8

Radiation

27 Bosses

8

Appearance

30 Gussets

9

Agency Approvals

30 Sharp Corners

9

Life Expectancy

32 Draft

9

Dimensional Tolerances

33 Holes and Cores

9

Processing

34 Undercuts

9

Production Quantities

34

9

Cost Constraints

36 Louvers and Vents

10

Assembly

37 Molded-In Threads

Rib Design

Slides and Cores

10 Thermoplastic Processing Methods

40 Lettering

10

Injection Molding

40 Tolerances

11

Extrusion

42 Bearings and Gears

12

Thermoforming

12

Blow Molding

13

Rotomolding

13 Optimizing Product Function 14

Consolidation

14

Hardware

14

Finish

15

Markings and Logos

15

Miscellaneous

15 Reducing Manufacturing Costs 15

Materials

16

Overhead

17

Labor

17

Scrap and Rework

17 Prototype Testing

3

Chapter 3 STRUCTURAL DESIGN

Chapter 4 DESIGN FOR ASSEMBLY

45 Structural Considerations In Plastics

83 Part Consolidation

46

Stiffness

84 Mechanical Fasteners

46

Viscoelasticity

85 Snap-Fit Joints

48

Stress-Strain Behavior

88 Welding and Bonding

50

Molding Factors

89

Ultrasonic Welding

51 Short-Term Mechanical Properties

90

Vibration and Hot-Plate Welding

51

Tensile Properties

91

Spin Welding

52

Tensile Modulus

91

Solvent and Adhesive Bonding

52

Tensile Stress at Yield

92 Retention Features

52

Tensile Stress at Break

92 Alignment Features

53

Ultimate Strength

94 Orientation

53

Poisson's Ratio

94 Expansion Differences

53

Compressive Properties

94 Tolerances

53

Flexural Modulus

53

Coefficient of Friction

54 Long-Term Mechanical Properties

Chapter 5 MACHINING AND FINISHING

54

Creep Properties

56

Stress Relaxation

97 Drilling and Reaming

56

Fatigue Properties

99 Tapping

58 Structural Design Formulas

99 Sawing

58

Use of Moduli

100 Punching, Blanking, and Die Cutting

59

Stress and Strain Limits

101 Milling

60

Uniaxial Tensile and Compressive Stress

101 Turning and Boring

61

Bending and Flexural Stress

102 Laser Machining

65

Shear Stress

103 Filing

66

Torsion

103 Sanding

67 Designing for Stiffness

103 Polishing and Buffing

67

Part Shape

104 Trimming, Finishing, and Flash Removal

70

Wall Thickness

71

Ribs

73 Long-Term Loading 76 Designing for Impact 78 Fatigue Applications 80 Thermal Loading

4

Chapter 6 PAINTING, PLATING, AND DECORATING

Chapter 7 MOLD DESIGN

105 Painting

121 Mold Basics

105

Types of Paints

121 Types of Molds

106

Paint Curing

124 Mold Bases and Cavities

106

Paint-Selection Considerations

125 Molding Undercuts

107

Spray Painting

128 Part Ejection

108

Other Painting Methods

130 Mold Venting

108

Masking

130

Parting-Line Vents

109

Other Design Considerations for Painting

131

Vent Placement

109 In-Mold Decorating

133 Sprues, Runners, and Gates

110 Film-Insert Molding

133

Sprues

111 Metallic Coatings

134

Runners

111

137

Runners for Multicavity Molds

112

Design Considerations for Electroplating

140

Gates

113

Molding Considerations for Electroplating

144

Other Gate Designs

145

Gate Optimization

147

Gate Position

114

Electroplating

Vacuum Metallization

115 115

Design Considerations for Vacuum Metallization EMI/RFI Shielding

115

Design Considerations for EMI/RFI Shielding

149 Hot-Runner Systems 149

Hot-Runner Designs

116 Printing

149

Hot-Runner Gates

118 Labels and Decals

151

Valve Gates

119 Texture

151 Thermal Expansion and Isolation 152

Flow Channel Size

153 Mold Cooling 154

Mold-Cooling Considerations

155

Cooling-Channel Placement

158

Cooling-Line Configuration

159

Coolant Flow Rate

160 Mold Shrinkage 162 Mold Metals 163 Surface Treatments 164 Mold Cost and Quality

APPENDICES

165 Index 169 Part Design Checklist

5

6

Chapter 1

PART DESIGN PROCESS: CONCEPT TO FINISHED PART

Many factors affect plastic-part design. Among these factors are: functional requirements, such as mechanical loading and ultraviolet stability; aesthetic needs, such as color, level of transparency, and tactile response; and economic concerns, such as cost of materials, labor, and capital equipment. These factors, coupled with other design concerns — such as agency approval, processing parameters, and part consolidation — are discussed in this chapter.

DESIGN PROCESS Like a successful play in football, successful plastic product design and production requires team effort and a well-developed strategy. When designing plastic parts, your team should consist of diverse players, including conceptual designers, stylists, design engineers, materials suppliers, mold makers, manufacturing personnel, processors, finishers, and decorators. Your chance of producing a product that successfully competes in the marketplace increases when your strategy takes full advantage of team strengths, accounts for members’ limitations, and avoids overburdening any one person. As the designer, you must consider these factors early in strategy development and make adjustments based upon input from the various people on the design team. Solicit simultaneous input from the various “players” early in product development, before many aspects of the design have been determined and cannot be changed. Accommodate suggestions for enhancing product performance, or for simplifying and improving the various manufacturing steps such as mold construction, processing, assembly, and finishing. Too often designs pass sequentially from concept development to manufacturing steps with features that needlessly complicate production and add cost.

Early input from various design and manufacturing groups also helps to focus attention on total product cost rather than just the costs of individual items or processes. Often adding a processing step and related cost in one area produces a greater reduction in total product cost. For example, adding snap latches and nesting features may increase part and mold costs, and at the same time, produce greater savings in assembly operations and related costs. Likewise, specifying a more-expensive resin with molded-in color and UV resistance may increase your rawmaterial cost, while eliminating painting costs. When designing and developing parts, focus on defining and maximizing part function and appearance, specifying actual part requirements, evaluating process options, selecting an appropriate material, reducing manufacturing costs, and conducting prototype testing. For the reasons stated above, these efforts should proceed simultaneously.

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DEFINING PLASTIC PART REQUIREMENTS Thoroughly ascertain and evaluate your part and material requirements, which will influence both part design and material selection. When evaluating these requirements, consider more than just the intended, end-use conditions and loads: Plastic parts are often subjected to harsher conditions during manufacturing and shipping than in actual use. Look at all aspects of part and material performance including the following.

Mechanical Loading

Carefully evaluate all types of mechanical loading including short-term static loads, impacts, and vibrational or cyclic loads that could lead to fatigue. Ascertain long-term loads that could cause creep or stress relaxation. Clearly identify impact requirements.

Temperature

Many material properties in plastics — impact strength, modulus, tensile strength, and creep resistance to name a few — vary with temperature. Consider the full range of end-use temperatures, as well as temperatures to which the part will be exposed during manufacturing, finishing, and shipping. Remember that impact resistance generally diminishes at lower temperatures.

8

Chemical Exposure

Plastic parts encounter a wide variety of chemicals both during manufacturing and in the end-use environment, including mold releases, cutting oils, degreasers, lubricants, cleaning solvents, printing dyes, paints, adhesives, cooking greases, and automotive fluids. Make sure that these chemicals are compatible with your selected material and final part.

Electrical Performance

Note required electrical property values and nature of electrical loading. For reference, list materials that are known to have sufficient electrical performance in your application. Determine if your part requires EMI shielding or UL testing.

Weather Resistance

Temperature, moisture, and UV sun exposure affect plastic parts’ properties and appearance. The end-use of a product determines the type of weather resistance required. For instance, external automotive parts such as mirror housings must withstand continuous outdoor exposure and perform in the full range of weather conditions. Additionally, heat gain from sun on dark surfaces may raise the upper temperature requirement considerably higher than maximum expected temperatures. Conversely, your requirements

may be less severe if your part is exposed to weather elements only occasionally. For example, outdoor Christmas decorations and other seasonal products may only have to satisfy the requirements for their specific, limited exposure.

Radiation

A variety of artificial sources — such as fluorescent lights, high-intensity discharge lamps, and gamma sterilization units — emit radiation that can yellow and/or degrade many plastics. If your part will be exposed to a radiation source, consider painting it, or specifying a UV-stabilized resin.

Appearance

Aesthetic requirements can entail many material and part-design issues. For example, a need for transparency greatly reduces the number of potential plastics, especially if the part needs high clarity. Color may also play an important role. Plastics must often match the color of other materials used in parts of an assembly. Some applications require the plastic part to weather at the same rate as other materials in an assembly.

Chapter 1

PART DESIGN PROCESS: CONCEPT TO FINISHED PART continued

In resins, custom colors generally cost more than standard colors, particularly for small-order quantities. For certain colors and effects, some parts may need to be painted or decorated in the mold. Depending upon the application, parts with metallic finishes may require painting, in-mold decorating or vacuum metallization. Surface finishes range from high-gloss to heavy-matte. Photoetching the mold steel can impart special surface textures for parts. Styling concerns may dictate the product shape, look, and feel, especially if the product is part of a component system or existing product family. Note all cosmetic and non-cosmetic surfaces. Among other things, these areas may influence gate, runner, and ejector-pin positioning. Many part designs must include markings or designs such as logos, warnings, instructions, and control labels. Determine if these features can be molded directly onto the part surface or if they must be added using one of the decorating methods discussed in Chapter 6.

Agency Approvals

Government and private agencies have specifications and approval cycles for many plastic parts. These agencies include Underwriters’ Laboratories (UL) for electrical devices, Military (MIL) for military applications, Food and Drug Administration (FDA) for applications with food and bodily-fluid

contact, United States Department of Agriculture (USDA) for plastics in meat and poultry equipment, and National Sanitation Foundation Testing Laboratory, Inc. (NSF) for plastics in food-processing and potable-water applications. Always check for compliance and approval from appropriate agencies. Determine if your part requires flame resistance in...